As one of melt-moldable fluororesins, an ethylene/tetrafluoroethylene copolymer (hereinafter sometimes referred to also as ETFE) is available. A melt-moldable fluororesin is excellent in heat resistance, chemical resistance, electrical insulating properties, flame retardancy, weather resistance and melt-moldability and thus is used as an insulating covering material for electric wires to be used for e.g. aircrafts, atomic power plants, automobiles, industrial robots, etc.
Such a melt-moldable fluororesin is produced in the form of fine fluororesin particles, by a polymerization method such as solution polymerization or suspension polymerization. The obtained fine fluororesin particles are granulated into fluororesin beads and then melt-molded in a pellet form to obtain fluororesin pellets. Such fluororesin pellets are used as a molding material to produce various products. The fluororesin pellets are excellent in handling efficiency at the time of the molding process for such various products.
By the way, if a fluororesin containing hydrogen and fluorine in the main chain, such as ETFE, is held at a high temperature for a long time, hydrofluoric acid (HF) may sometimes be released from the main chain. Further, as disclosed in Non-patent Document 1, it is known that ETFE generates a gas containing hydrofluoric acid at the melt-molding temperature.
Hydrofluoric acid shows very high solubility in water, and therefore, hydrofluoric acid is not substantially contained in fluororesin beads obtained by granulating fine fluororesin particles obtained by polymerization, by using an aqueous medium.
However, at the time of melt-molding the fluororesin beads in a pellet form, hydrofluoric acid may sometimes be released from the main chain of the fluororesin. Therefore, hydrofluoric acid may sometimes be contained in the fluororesin pellets. If hydrofluoric acid is contained in the fluororesin pellets, hydrofluoric acid will be released from the fluororesin pellets to a gas phase as the time passes. Accordingly, in a case where the fluororesin pellets are stored in a closed place for a long period of time, or in a case where they are stored at a high temperature in summer time, an odor of the acid may sometimes be smelled.